1. Field of the Invention
The present invention relates to a transmission electron microscope having means for supplying gas into the specimen chamber.
2. Description of Related Art
One example of transmission electron microscope having means for supplying gas into the specimen chamber is schematically shown in FIG. 1. The microscope has an electron gun chamber 1 in which an electron gun 2 emitting an electron beam is mounted. The microscope further includes an illumination lens chamber 3 in which condenser lenses 4 and scan coils 6 are mounted. The condenser lenses 4 converge the electron beam. The scan coils 6 scan the converged beam over the surface of a specimen 5 in two dimensions (in X- and Y-directions).
The microscope further includes a specimen chamber 7 in which the specimen 5 is disposed. The specimen 5 is mounted to a side entry specimen holder (not shown), which in turn is mounted between the upper polepiece 8 and lower polepiece 9 of an objective lens. Each of the upper polepiece 8 and lower polepiece 9 is provided with an electron beam passage hole. Orifices 11a and 11b are mounted at higher and lower positions, respectively, in the electron beam passage hole formed in the upper polepiece 8 of the objective lens. Similarly, orifices 11c and 11d are mounted at higher and lower positions, respectively, in the electron beam passage hole formed in the lower polepiece 9 to hinder an environmental gas introduced into the specimen chamber 7 from flowing into other portions. A vacuum gauge (not shown) for measuring the degree of vacuum in the specimen chamber 7 is also mounted in the specimen chamber 7.
The electron microscope further includes an imaging lens chamber 12 in which an intermediate lens 16a and a projector lens 16b are mounted to cause the electrons transmitted through the specimen 5 and converged by the objective lens to be magnified and projected onto a fluorescent screen 14 disposed in an observation chamber 13.
The fluorescent screen can be retracted from the optical axis O of the electron beam by fluorescent screen-retracting means (not shown). A CCD camera 15 can detect the magnified image. The output signal from the camera 15 is fed to a controller 18 via an A/D converter 17. The controller controls various components of the transmission electron microscope and performs various calculations.
A display device 19 has a display screen on which an image of the specimen (e.g., a transmission electron image of the specimen) is displayed based on the image signal from the controller 18. Input devices 20, such as a computer mouse and a keyboard, are connected with the controller 18.
A vacuum pump 31 is used to evacuate the inside of the electron gun chamber 1. Another vacuum pump 32 evacuates the insides of the illumination lens chamber 3, imaging lens chamber 12, and observation chamber 13. A further vacuum pump 33 evacuates a space surrounded between the orifice 11a in the upper polepiece 8 of the objective lens and the orifice 11b via an evacuation tube 34. The vacuum pump 33 also evacuates a space between the orifice 11c in the lower polepiece 9 of the objective lens and the orifice 11d via an exhaust pipe 35. An additional vacuum pump 36 evacuates the inside of the specimen chamber 7.
The environmental gas introduced into the specimen chamber 7 is stored in a gas cylinder 37. The gas cylinder 37 is opened and closed by a gas valve 38. The flow rate of the environmental gas from the gas cylinder 37 is adjusted by a gas flow rate controller 39. A gas nozzle 40 is used to introduce the environmental gas from the gas cylinder 37 into vicinities of the specimen 5. The portion of nozzle 40 is located inside the specimen chamber and the front end of the gas nozzle 40 is located immediately above the specimen 5.
Where a transmission electron image should be obtained by the transmission electron microscope of this structure, the electron beam from the electron gun 2 is converged by the condenser lenses 4 and passes through the upper polepiece 8 of the objective lens. Then, the beam is focused onto the surface of the specimen 5.
At this time, the electrons transmitted through the specimen are passed through the lower polepiece 9 of the objective lens, intermediate lens 16a, and projector lens 16b. As a result, the cross section of the beam is magnified in turn. A magnified image is displayed on the fluorescent screen 14.
The fluorescent screen 14 is retracted by the fluorescent screen-retracting means (not shown) and the magnified image is detected by the CCD camera 15. The output signal from the camera 15 is furnished to the controller 18 via the A/D converter 17. The transmission electron signal is processed in a given manner by the controller 18 and sent to the display device 19. Consequently, a transmission electron image is displayed on the display screen of the display device.
In this transmission electron microscope, the environmental gas may sometimes be blown against around the specimen 5 to make in situ observation of the process of spontaneous reactions between the specimen 5 and the environmental gas.
Where such an image observation is performed, a given amount of environmental gas is supplied around the specimen 5 inside the specimen chamber 7 via the gas nozzle 40 from the gas cylinder 37 by opening the gas valve 38 and sending a flow rate drive signal from the controller 18 to the gas flow rate controller 39 under instructions from the controller 18. This induces a reaction with the specimen.
At this time, the orifices 11a, 11b, 11c, and 11d located at higher and lower positions in the electron beam passage holes formed in the upper polepiece 8 and lower polepiece 9 of the objective lens suppress the environmental gas in the specimen chamber 7 from flowing into the illumination lens chamber 3 and imaging lens chamber 12. Consequently, the degrees of vacuum in the chambers are maintained at levels normally used during observation.
After desired reactions, the supply of the environmental gas to the specimen 5 is once halted. The inside of the specimen chamber 7 is evacuated by the vacuum pump 36 and then the electron beam irradiation is resumed. An image of the specimen can be displayed on the display device 19 under a vacuum environment (see JP8-329876).
Where an atomic array of a specimen is observed at high resolution with an electron microscope, dark field imaging, such as the technique of high-angle annular dark-field (HAADF) imaging, is adopted. In this method, an annular detector is prepared as a detector for dark field microscopy. Only the portions of the electrons transmitted through the specimen which are scattered at high angles and which thus impinge on the annular portion of the detector are detected by this annular portion. A dark field image based on electrons detected by the detector is displayed on a display device. The dark field image obtained by this method suffers from less diffraction effects. It is possible to clearly observe a dark field image whose contrast is seen to vary with atomic number.
However, where this method of observation is applied to a transmission electron microscope having means for supplying gas into the specimen chamber, electrons passing through the specimen at high scattering angles as described above are blocked off by the orifices 11c and 11d fixed to the lower polepiece 9 of the objective lens. Hence, there is the danger that these electrons cannot reach the detection surface of the dark field detector (not shown) disposed in the observation chamber 13.
The process of reaction between the specimen and the environmental gas depends on the pressure of the gas around the specimen. If the orifices are placed at rest, the diameters of the openings of the orifices are fixed. Consequently, the maximum allowable pressure of the environmental gas that can be introduced into the specimen chamber 7 is fixed.